The invention relates to broadband access systems and particularly to a network device for DSL (Digital Subscriber Line) access systems.
Currently, a primary broadband service to the homes is high-speed Internet access, which has turned the home computers into “always-on” home appliances. Public telephone networks usually utilize modems which transmit data at a rate of 56 kbps, for instance. In the telephone network the modems use the same frequency range as the telephones to transmit speech, the range being about 400 Hz to 4 kHz. However, the line between a switching center and a user, i.e. a subscriber line, allows frequencies much higher than 0 to 4 kHz to be used and thus more data to be transmitted. This limitation to 4 kHz is due to filters which are located in switching centers and filter additional frequencies from a signal supplied to the backbone network. Actually, in the subscriber line, typically made of a twisted copper pair, frequencies as high as several MHz can be transmitted.
DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth information to e.g. homes and small businesses over ordinary copper telephone lines. Digital Subscriber Line is a technology that assumes that digital data requires no change into analogue form and back. Digital data is transmitted to a subscriber directly as digital data and this allows a much wider bandwidth to be used for transmitting the data than in traditional telephone systems. There are several DSL interfaces available, such as ADSL (Asymmetric DSL), SHDSL (Single-pair High-speed DSL), and VDSL (Very High data rate DSL).
An example of an xDSL system is illustrated in FIG. 1. A subscriber's end-user equipment (such as a router, modem, or network interface card, NIC) 1 located at premises owned or controlled by a customer using network services is often referred to as customer premises equipment, CPE. On the network side, multiplexing equipment 2 that may contain a high concentration of central office splitters, xDSL modems, and other electronic equipment to connect traffic to a data network is often referred to as a digital subscriber line access multiplexer, DSLAM. A DSLAM delivers exceptionally high-speed data transmission over existing copper telephone lines and controls and routes the digital subscriber line (xDSL) traffic between the CPEs and the network service provider's network, such as an ATM/IP network.
One of the challenges to DSL operators is to select and deploy a correct deployment model for the broadband services. Currently, operators have selected different models: few of the operators use point-to-point protocol (PPP) over ATM, some use 1483 routed, and most of the operators use PPP over Ethernet or 1483 bridged. Operators have different criteria for selecting the deployment model. Their choice may be based for example on the number of public IP addresses needed, the price of the CPE or CPE installation and wholesale model. One of the key criteria today is the Layer 2 technology in use in the access network.
The standard specifies ATM to be used in the transport layer on top of the physical ADSL layer. If ATM is used also on the network side, the connections from DSL subscribers can easily be established and switched using ATM PVCs (Permanent Virtual Connections) and IP level intelligence is not required. An ATM access network basically supports all deployment models but introduces unnecessary overhead and is heavy to manage, whereas Ethernet based access networks cannot support deployment models where ATM is present. With Ethernet networks, the traditional ATM switching model needs to be replaced by packet switching, which can be carried out either on an Ethernet layer (layer 2) or an IP layer (layer 3), depending on the chosen deployment model.
Both access technologies have their limitations as well as advantages, and supporting all the possibilities creates more requirements for the DSLAMs. A DSLAM must support simultaneous feeds from ATM and IP interfaces, while it should at the same time provide the required service levels over both QoS (ATM) and CoS (IP) connections and flows simultaneously. ATM QoS (Quality of Service) enables service providers to fully utilize available bandwidth while managing multiple classes of service, such as Constant Bit Rate (CBR), Variable Bit Rate real-time (VBR-rt), and Variable Bit Rate non-real-time(VBR-nrt). IP CoS provides the rules as to how to manage IP flows. Typical IP Class of Services classes include Expedited Forwarding (EF), Assured Forwarding (AF), and Default Forwarding (DF).
One approach to embody a DSLAM is illustrated in FIG. 2. As packets and cells ATM and IP traffic flows enter the DSLAM, they may pass through four major steps to ensure that they are handled appropriately relative to the type of information or services they are carrying: classification and marking, policing, queuing, and scheduling and shaping. These operations can be performed using a network processor that is able to provide wire-speed performance with a flexible programming environment and simultaneously support many different communication protocols. One such processor is a PayloadPlus traffic management network processor from Agere Systems. In a DSLAM application, the network processor can be configured to function as a centralized switch and can feed multiple DSL line cards, each with many ports. The internal logic provides high-level control of network processor scheduling functions, i.e. traffic manager (TM) and traffic shaper (TS) compute engine (CE). A TM compute engine (CE) in a network processor offers fully programmable traffic management capabilities, including buffer management policies, through a reduced and optimized C instruction set. To enforce a buffer management policy, a short script is executed on the buffer management-processing engine. The traffic management script determines whether or not to discard or allow traffic to be scheduled. This decision is based on the user programmed discard policy. In addition, a network processor has the facilities to schedule cell and frame-based traffic using a number of scheduling algorithms. The processor's configuration, the traffic shaper (TS) compute engine (CE), and the programmable queue definitions define the traffic shaping. For example, the traditional ATM constant bitrate (CBR), variable bitrate (VBR) and unspecified bitrate (UBR) schedulers are programmable in the network processor. The traffic shaper compute engine executes a program defined at every scheduling event. The traffic shaper may also be used to support class of service queue selection for frame-based traffic using algorithms such as weighted round robin; variable-bitrate cell-based traffic scheduling using dual leaky bucket algorithms; and frame-based, smoothed, deficit-weighted, round robin scheduling by tracking rate credits for each queue.
There may be several queues and traffic shaping schedulers for each logical output port (mapped to an xDSL port, for example) in a traffic shaper computer engine. The transmission of cells or blocks from the queues to the logical output port is scheduled based on a so-called main scheduling time wheel principle. The logical output port has a predetermined port rate, e.g. 10 Mbps. QoS (ATM) and/or CoS (IP) connections and flows share the port rate. For this purpose, a queue interval is dynamically determined for each queue. The queue interval defines the number of main scheduling time slot to be skipped before a next block or cell is transmitted from a specific queue. If the number of time slots to be skipped is zero, also the next block is transmitted from the same queue unless the queue is empty. This means that the queue rate is 10 Mbps. If the number of time slots to be skipped is 1, the traffic shaper CE next transmits from another queue having the next highest priority, before transmitting again from the first queue. This means that the queue rate is 5 Mbps. Similarly, when the number of time slots to be skipped is 2 or 3, a queue rate of 3.33 Mbps or 2.5 Mbps, respectively, is obtained.
A problem with this approach is inadequate granularity in QoS provisioning when using generic QoS/CoS traffic shaping: For a 10 Mbps port we only get 10, 5, 3.3, 2.5, 2, 1.7, 1.3 and 1 Mbps as queue rates. Also, there may easily be significant latency in the scheduling parameters sinking in, so the QoS provisioning is not very precise. Still further, ingenious scripting is required to facilitate a mix of ATM and packet data traffic, due to a very limited scripting interface. The traffic schedulers are controlled via programmable scripts that are executed for each block. In the Agere's network processor, this means that the script must be executed within 22 clock cycles (RSP global pulse) which severely limits the complexity that can be imposed in the algorithms.